1. Field of the Invention
The present invention relates to a plastic sheet for rear projection screens to be used in rear projection television sets, etc.
2. Description of the Related Art
Heretofore known is a rear projection sheet comprising a plurality of sheet members such as a lenticular lens sheet, a Fresnel lens sheet, a prism lens sheet, a protect shield, etc. For such rear projection screens, lenticular lens sheets having a multi-layered laminate structure are being popularized (see Japanese Patent Laid-Open No. 61120/1993, etc.). The multi-layered lenticular lens sheet improves light utilizing efficiency, as making it possible to concentrate a light-diffusing agent at its layer most adjacent to viewers. In case where an antistatic agent is added thereto, it may be in that layer most adjacent to viewers. To the multi-layered lenticular lens sheet, predetermined additives can be so added that they are well concentrated at the layers requiring them. Accordingly, the amount of the light-diffusing agent, the antistatic agent and other additives to be added to one lenticular lens sheet of that type can be reduced, and the raw material expenses for such additives can be reduced.
The recent tendency in the art of rear projection screens is toward reducing the thickness of the sheet members constituting them, for evading so-called xe2x80x9cghostsxe2x80x9d to be caused by unintended reflected rays formed inside the plurality of sheet members, but such thin sheet members are readily cracked. To solve the problem, a rubber material is often added to the sheet members to improve their impact resistance.
A rear projection screen generally comprises a combination of two or more plastic sheets that differ from each other in the thickness and the material. In this, the constituent sheets must not have any space therebetween, and they are warped so as to have arched vertical sides when they are actually fitted to a television set. Briefly, the sheets are fitted to a television set with their outer peripheries being pressed against it, and they are airtightly adhered to each other owing to their elasticity. To warp the plastic sheets in that manner, in general, they are put into a warping mold and heated at a predetermined temperature. However, when two or more multi-layered plastic sheets with a rubber material of an impact resistance improver being added thereto are warped according to such an ordinary method of putting them in a warping mold followed by heating them therein, they could not be well warped to such a degree that they have no space therebetween when combined into a rear projection screen. To solve the problem, the plastic sheets will be warped in a warping mold having a smaller radius of curvature than usually. However, the method of warping plastic sheets in such a warping mold having a smaller radius of curvature is problematic in that the warped plastic sheets are locally deformed or are unfavorably swollen or waved. In addition, the warping mold having a smaller radius of curvature shall be deep and large-sized, and is therefore difficult to handle. With the warping mold of that type, the productivity of plastic sheets is extremely low.
The present invention has been made in consideration of the problems noted above, and its object is to provide a plastic sheet for rear projection screens which can be well warped in any ordinary method of warping plastic sheets not containing a rubber material, with no failure of unfavorably deformed, swollen or waved sheets.
The plastic sheet for rear projection screens of the invention that solves the problems as above has a two or more multi-layered structure, in which the coefficient of linear expansion (at 20xc2x0 C.) of one layer is higher by at least 1.0xc3x9710xe2x88x926 mm/(mmxc2x7xc2x0C.) than that of any other layer. In one embodiment of the plastic sheet, the layer of which the coefficient of linear expansion is higher than that of any other layer contains a fine particulate rubber material having a diameter of from 0.1 to 100 xcexcm to such a degree that the rubber material content thereof is higher by at least 0.5% by weight than that of any other layer.
The plastic sheet for rear projection screens is so warped that the layer of which the coefficient of linear expansion is higher than that of any other layer therein is concaved inside the sheet. In order that the plastic sheet can have higher impact resistance, the thickness of the layer of which the coefficient of linear expansion is higher than that of any other layer therein is preferably at least {fraction (1/20)} of the overall thickness of the plastic sheet.
The plastic sheet for rear projection screens of the invention is used as a lenticular lens sheet, a Fresnel lens sheet, etc. It is also usable for any other sheet members such as a prism sheet, a protect shield and others in rear projection screens.